The present invention relates to the annealing of a newly formed glass ribbon and particularly relates to a method and apparatus for sensing the temperature at a critical portion of the cooling cycle at which the ease of cutting the ribbon and the likelihood of glass breakage is determined. The present invention is particularly useful to monitor the temperature of a moving ribbon of float glass in the region or main portion of the ribbon that is usually cut into rectangular or square blanks of commercially acceptable glass from the ribbon.
In the past, glass surface temperature measurements during annealing have been performed by thermocouples or temperature sensing elements spaced from the glass surface. Using temperature sensing elements spaced from the glass surface avoided scratching of the glass surfaces. However, other problems such as the sensing of extraneous radiation not associated with the temperature of the glass being monitored tended to cause errors in the sensed temperature.
Typical examples of patents providing non-contacting temperature sensing devices for moving glass ribbons are U.S. Pat. No. 2,912,862 to Machler et al and U.S. Pat. No. 3,600,947 to Farabaugh.
U.S. Pat. No. 3,500,548 to Mitsuno discloses the use of a pair of graphite electrodes positioned with a ribbon forming apparatus and adapted to contact the edge portion of a newly formed ribbon for the purpose of determining the presence or absence of the edge at an inspection station in a method of detecting and measuring the width of the ribbon as it passes the detection station. When the ribbon is narrower than the range of width to be detected, the electrodes do not contact any glass ribbon. When the ribbon width is greater, an edge portion of the ribbon is in contact with one or both of the electrodes mounted for contact by the ribbon edge portion. However, the edge detection apparatus of the Mitsuno patent is not used for determination of temperature of the ribbon.
U.S. Pat. No. 3,957,475 to Schwenninger and Welton involves determining the temperature of the ribbon along a line of movement at a given distance transversely inward from the edge of the ribbon when the ribbon is cooling through a critical range of temperatures. The line of movement is laterally outside a line along which the ribbon is to be cut along its length after the ribbon is annealed and usually represents a portion of the ribbon that would otherwise be unused, except perhaps as cullet, particularly when the ribbon is of float glass which usually has edge portions of inferior optical quality. The latter patent provides a particular apparatus to insure that the line along which the temperature is sensed is a desired distance from the glass edge and that the temperature sensing element is in heat conductive relation with the surface of the ribbon along the line of temperature measurement.
If the surface temperature of the glass ribbon along its edge portions is properly controlled, the ribbon can be subsequently cut easily, particularly along a line extending longitudinally of the ribbon path through the annealing lehr. Furthermore, controlling the rate at which the glass ribbon cools through the critical range of temperatures is a factor in controlling spontaneous breakage or splitting of the ribbon both longitudinally and laterally of its path of movement through the annealing lehr.
The Schwenninger and Welton patent uses glass temperture sensing means in heat conducting relation to the ribbon through heat conducting elements that contact the moving glass ribbon along a portion of said ribbon that would be wasted anyway without causing damage to the optical properties of the glass sheets that are subsequently cut away from the ribbon, a factor that discouraged the use of glass contacting temperature sensing means in the past.
The Schwenninger and Welton patent provides means for detecting the temperature of the glass ribbon at a fixed distance adjacent each of the opposite longitudinal side edges thereof in the region where the ribbon cools through a critical temperature zone. The apparatus disclosed therein provides temperature sensing means disposed in heat-conducting relation with a surface of the ribbon and is supported by means that is in slidable relation with the ribbon surface. While such sliding relation has been avoided in the past, the Schwenninger and Welton patent involves making such sliding contact along the edge portions of the ribbon that must be trimmed anyway, so that any surface marking resulting from such sliding contact does not harm the usable portion of the ribbon, yet the accurate temperature readings obtained can be correlated with the cooling rate of the usable portion of the ribbon.
The inventions disclosed in the prior art, despite the ability to avoid marring of the pristine surface of a newly formed glass ribbon has limited operators to either try to infer and control glass temperatures from data obtained from a combination of ambient air temperatures (via thermocouples above the glass), and tunnel temperatures (also via thermocouples), or measuring actual glass temperatures in regions other than the critical regions where it is necessary to know the exact glass temperatures with optical radiation pyrometers.
The first technique, using ambient and tunnel temperatures, provides at best only relative information concerning glass temperatures. With annealing dependent on such things as viscosity, which in turn relates to absolute temperatures in an exponential fashion, relative temperature measurements are less than ideal. Previously, temperatures at critical locations have been estimated by developing empirical relationships that define energy distribution, cooling air flows, etc., in terms of various parameters such as ribbon speeds, thickness, etc. This is a complex and cumbersome procedure that, at best, has uncontrolled parameters introducing variations in the annealing process that often go undetected until manifest in the finished product as undesirable stress profiles, poor cutting, or worse, cross breaks or splits. These in turn dictate another round of empirical adjustments that are time consuming to develop and still lack accuracy because they cannot possibly be adjusted promptly in response to changes in parameters of lehr atmosphere and glass characteristics that dictate such adjustments.
The development of sensitive optical radiation pyrometers was heralded a few years ago as the solution to the aforesaid annealing woes. Much time and effort have gone into programs at float glass production units to make them a viable tool. After working with them for several years, further improvements in accuracy of temperature measurements were still needed. At glass-annealing temperatures, with the necessary optical filtering, the output is so small that signal resolution and signal-to-noise ratios become major problems. Furthermore, in spite of the best efforts to control the annealing process based on temperature determinations, intolerable problems were met with signals signifying glass temperature drifting to such an extent that it was difficult, if not impossible, to determine absolute glass temperatures by the empirical methods developed in the prior art.